bfd: fix potential missing seek

bfd: fix potential missing seek

If a file was closed by cache and then bfd_open_file was called followed
by e.g. bfd_seek to the original position, the seek would be optimized out
while the real file position was still zero (as fopened).  I added
`bfd_io_force` to force the seek at next IO occasion.

This could lead e.g. to appearence of a corrupted object in ld:
  symbol number 0 uses unsupported binding of 6
  or invalid string offset #X >= #Y for section

Signed-off-by: Roman Kapl <code@rkapl.cz>